The present invention is directed generally to optical systems. More particularly, the invention is directed toward optical transmission systems including amplifying devices, such as optical amplifiers.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has accelerated demand for high capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to increase the capacity of transmission systems to meet the increasing capacity demands placed on their network infrastructure.
In WDM transmission systems, pluralities of distinct information signals are carried using electromagnetic waves having different wavelengths in the optical spectrum, typically using infrared wavelengths. Each information carrying wavelength can include multiple data streams that are time division multiplexed (“TDM”) together into a TDM data stream or a single data stream.
The pluralities of information carrying wavelengths are combined into a “WDM” optical signal that is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of the network compared to space division multiplexed (“SDM”), i.e., single channel, systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems were not initially deployed, in part, because of the high cost of electrical signal regeneration/amplification equipment required to compensate for signal attenuation for each optical wavelength throughout the system. However, the development of the erbium doped fiber amplifier (EDFA) provided a cost effective means to amplify optically multiple optical signal wavelengths in the 1550 nm range. In addition, the 1550 nm signal wavelength range coincides with a low loss transmission window in silica based optical fibers, which allowed EDFAs to be spaced further apart than conventional electrical repeaters/regenerators.
Optical amplifiers are deployed periodically, e.g., 40–120 km, throughout the optical system to compensate for attenuation that incurs in a span of optical fiber preceding the amplifier. The amplifiers are operated so that the gain provided by the optical amplifier compensates, or substantially compensates, for the loss in each span. As a result, no net loss or gain of signal power occurs in each span, i.e. Amplifier Gain≅Span Loss, which is referred to as transparent operation.
The use of EDFAs essentially eliminated the need for, and the associated costs of, electrical signal repeater/regeneration equipment to compensate for signal attenuation in many systems. The dramatic reduction in the number of electrical regenerators in the systems, made the installation of WDM systems in the remaining electrical regenerators a cost effective means to increase optical network capacity.
WDM systems have quickly expanded to fill the limited amplifier bandwidth of EDFAs. New erbium-based fiber amplifiers (L-band) have been developed to expand the bandwidth of erbium-based optical amplifiers. Also, new transmission fiber designs are being developed to provide for lower loss transmission in the 1400–1500 nm and 1600–1700 nm ranges to provide additional capacity for future systems.
In addition, Raman fiber amplifiers (“RFA”) have been investigated for years and are now being commercially deployed and operated in a network. RFAs offer the potential to exploit a substantial portion of the optical waveguide transmission capacity
While optical amplifiers have provided significant benefits by eliminating the need for numerous electrical regenerators, optical amplifiers do have a shortcomings. For example, optical amplifiers often do not provide uniform amplification, or gain, profile over the signal wavelength range. As such, optical amplifiers often will be deployed in combination with gain flattening filters, which provide wavelength specific filtering to produce a more uniform bring about more uniform gain.
In addition, the gain profile of the optical amplifier can vary depending upon the amount of gain, or gain, being provided by the amplifier. In operating system, the amplifier gain is set to compensate signal power attenuation that occurs in a fiber span preceding the amplifier. The attenuation in each span, i.e., the span loss, generally varies from span to span in a system; therefore, the optical amplifiers have to be operated at different gains corresponding to the span loss. However, operation at different gain can introduce gain profile variations that result in signal power variations, which can degrade system performance.
While it is possible to design gain flattening filters and amplifiers for specific span losses, individualized amplifier and filter designs generally are not feasible from a commercial standpoint. As such, amplifiers generally are designed for a nominal gain and gain flattening filters are designed based on that nominal gain. When the amplifiers and filters are deployed in the system, operation of the amplifiers at gains other than the nominal gain will introduce signal power variations into the system.
Alternative designs have been proposed, in which the amplifiers are operated at the designed nominal gain and a variable attenuator is provided proximate the amplifier to introduce additional attenuation into the span. The variable attenuator is controlled, such that the variable attenuator loss plus the span loss is equal to the nominal gain of the amplifier.
The variable attenuator configurations allow the operation of optical amplifiers at designed gains allowing for more uniform gain profiles. However, the introduction of excess gain balanced by excess attenuation introduces additional noise into the system that also degrades system performance. In addition, these alternative designs require that the amplifier be designed to provide high gain that can be attenuated to accommodate various span loss, which can increase overall amplifier and system costs.
The development of higher performance, lower cost communication systems depends upon the continued development of higher performance components and subsystems for use in the system. It is, therefore, essential that optical systems and optical amplifiers be developed having increased performance capabilities to meet the requirements of next generation optical systems.